1. Field of Invention
The present invention relates to signal processing in GPS receivers. In particular, the present invention relates to a method and apparatus for delivering satellite data to GPS receivers to enable a GPS receiver to acquire and lock on to GPS satellite signals in low signal strength environments (e.g., indoors).
2. Description of the Background Art
Conventional GPS receivers require an inordinate amount of time to acquire and lock onto the satellite signals. Then, once locked, a GPS receiver extracts telemetry data (almanac and ephemeris) from the signal. From these data the GPS receiver can calculate information that enhances its ability to lock onto the satellite signal. A relatively high signal strength satellite signal is necessary to enable the system to achieve an initial lock. Once the GPS signal is acquired, the signal strength must remain high while the almanac and/or ephemeris data is extracted from the satellite signal. Any severe attenuation of the signal can cause a loss of lock and the signal will require re-acquisition. As such, the system has an inherent circularity that makes it difficult or impossible for GPS receivers to acquire signals in low signal strength environments.
To aid initial acquisition of the satellite signal, many GPS receivers store a copy of the almanac data, from which the expected Doppler frequency of the satellite signal can be calculated. Several techniques have been developed to calculate useful information at a separate GPS receiver and then transmit this data to another GPS receiver. U.S. Pat. No. 6,064,336, issued May 16, 2000, collects almanac data at a separate GPS receiver, then transmits the almanac data to a mobile receiver. The mobile receiver then uses the almanac data to compute the expected Doppler frequency of the satellite signal, thus aiding in initial signal acquisition.
The advantage of receiving the almanac is that each GPS satellite repeatedly transmits a complete almanac containing orbit information for the complete GPS constellation, thus a single GPS receiver, tracking any satellite, can collect and propagate the almanac for all satellites in the constellation. The disadvantage of using the almanac is that it is a fairly rough model of the satellite orbit and satellite clock errors, thus the almanac is only useful in reducing the frequency uncertainty and cannot be used to enhance receiver sensitivity by reducing the search window of code-delay uncertainties.
If a GPS receiver had a complete set of ephemeris data for all satellites in view, before said receiver attempted to lock onto those satellites, the receiver would have significantly improved acquisition times and enhanced sensitivity. This is because the ephemeris data contains an accurate description of the satellite position, velocity, and clock errors; and the GPS receiver can use this data to increase its sensitivity by reducing significantly the search windows for frequency uncertainty and code-delay uncertainty. The disadvantage of the ephemeris is that each satellite only transmits its own ephemeris; thus a single GPS receiver cannot collect and propagate ephemeris for all the satellites in the constellation.
Therefore there is a need in the art for a GPS receiver system that propagates satellite ephemeris for all satellites in the constellation, thereby enhancing the speed of acquisition and signal sensitivity of mobile receivers.
The invention comprises a method and apparatus for distribution and delivery of the Global Positioning System (GPS) satellite ephemeris using a communication link between a central site and a wide area network of GPS receivers. The wide area network of GPS receivers collects the ephemeris data that is transmitted by the satellites and communicates the data to the central site. The central site delivers the ephemeris to the mobile receiver. The mobile GPS receiver uses the delivered data to enhance its sensitivity in two ways. First, the data allows the receiver to detect very weak signals that the receiver would not ordinarily be able to detect, and second, the GPS receiver does not have to track the satellite signals for very long before a position can be calculated.
In one embodiment of the invention, the satellite ephemeris data is retransmitted without manipulating the data in any way. The GPS receiver may then use this data exactly as if the receiver had received the data from the satellite. In another embodiment, a satellite pseudo-range model is computed at the central site from the ephemeris data, and this pseudo-range model is transmitted to the GPS receiver. The pseudo-range model has the characteristic that the model is more concise than the complete ephemeris. As such, the GPS receiver does not have to perform as many calculations when using the pseudo-range model as when using the complete ephemeris.